Semiconductor gas sensors offer a low cost means of detecting gases with high sensitivity. However, they react to a number of gases and it is therefore difficult to discriminate between these in applications where more than one of these gases can be present.
The present invention addresses this problem in a number of ways used either singly or in combination and which can also significantly increase the sensitivity of the gas detection system in single gas applications.
The first of these is to use two or more semiconductor gas sensors of different characteristics, for example N type and P type sensors. Each sensor type responds differently to a particular gas, so one may estimate whether two or more gases are present by determining the relative response on each of the two or more sensors and comparing the relative response to the known relative responses to individual gases. To determine relative responses with maximum confidence, it is important to control the heater temperatures of each sensor precisely, as is done in the present invention.
The second technique is to scan or to modulate temperatures of the gas sensing elements over a wide temperature range and to detect the responses of the sensors at a number of carefully controlled temperatures. Semiconductor gas sensors respond to different gases in different ways across a range of temperatures. The response reaches a maximum value at a temperature that is characteristic for each gas. De-convolving the composite temperature response into the component for each constituent gas in a gas mixture thereby detects both the existence of different gases and their concentrations. The second technique can be combined with the first technique to further improve the results.
The third element of the approach is to use a modulated signal to measure the sensor response as a complex impedance, rather than the traditional resistance. This both improves the selectivity as determined by the other methods and allows a range of signal processing techniques to be used to improve noise rejection, and hence to increase effective sensitivity. A further improvement in the discrimination between gases in a mixture can be obtained by comparing the complex responses at a number of frequencies and comparing them with the known response to different gases.